The present invention is directed toward communications systems, and more particularly, toward a system and method for transmitting control information between a control unit and at least one sub-unit of at least one cascaded unit coupled to the control unit.
Certain communications systems are used in buildings, for example, airports, public buildings, corporate offices, etc. to provide cellular service to building occupants as radio signals often do not penetrate the structure of many buildings. One such communication system is the Ericsson RBS 884 Pico Base Station, where control and voice information is transmitted between a control and radio interface unit and a group of, for example, four radio heads typically located within a particular building structure using a T1 communications link. The Ericsson RBS 884 Pico Base Station may support up to eleven radio heads. The control information is transmitted within a T1 frame as High-level Data Link Control (HDLC) frames including a Link Access Procedure on the D-Channel (LAPD) format. Each radio head includes 4 radio transceivers, where each transceiver is able to service 3 mobile terminals, for example, cellular telephones.
A T1 frame comprises 24 time slots of information, 6 consecutive time slots of information for each radio head serviced by the control and radio interface unit. Each set of 6 consecutive time slots contains 2 control time slots of information, time slots 1 and 4, for transmitting control information between the control and radio interface and the transceivers within the respective radio head. Time slot 1 services the first and second transceivers of the radio head, and time slot 4 services the third and fourth transceivers of the radio head. The set of 6 consecutive time slots further includes 4 voice time slots, time slots 2,3,5 and 6, where each voice time slot is used to transmit voice information between the control and radio interface and a corresponding transceiver of a radio head.
As mentioned, the control time slots transmit control information between the control and radio interface and a particular radio head using an HDLC frame in LAPD format. The HDLC frame includes an address field having a Terminal Endpoint Identifier (TEI) location identifying a particular transceiver for which the control information is intended in the case of a downlink message from the control and radio interface unit and the radio head, and identifying a particular transceiver from which the control information is sent in the case of an uplink message from the radio head to the control and radio interface unit.
The control information transmitted is a minimum of 8 bytes of information in an HDLC frame between the control and radio interface unit and a radio head. Each time slot of the T1 frame contains 1 byte of information to be transmitted between the control and radio interface unit and the radio heads. Thus, where it is desired to transmit control information in an HDLC frame between the control and radio interface and a first transceiver of the first radio head, the control information is placed in the HDLC frame of the first time slot for the first radio head, specifying the first transceiver in the TEI location. However, as only one byte of the HDLC frame is transmitted per T1 frame, transmission of the entire HDLC frame requires a minimum of eight T1 frames. This is the case even where other transceivers, and their corresponding time slots in the T1 frame, are not in use.